It is widely accepted that active galactic nuclei (AGN) are powered by the accretion of matter into a supermassive black hole (BH). [1]} and [2]} are standard accretion disc models that are frequently used to explain the accretion process in these objects.
Since AGN accretion discs cannot be spatially resolved with ordinary telescopes, we have to rely on techniques other than imaging to measure their size.
An example of this is gravitational microlensing due to stellar-mass objects, such as stars in the lensing galaxy, which results in flux fluctuations. Their amplitude depends on the size of the emitting source. Thus, the results from optical microlensing observations can be used to measure the accretion disc size. Using this technique, disc sizes appear to be systematically larger than the standard model predictions by a factor of \(\sim 2-4\)  [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}. Various physical explanations have been proposed over the past few years to explain this difference [10]}, [11]}, [12]}, [13]}, [14]}.
